Tape data storage is typically used for backup, archival, and/or sequential data processing purposes. Examples of sequential processing are batch updating of master files or data mining where queries are aggregated in one complete sequential scan of the data. The random retrieval of data from a sequentially recorded pattern has been relatively simple, moving longitudinally along the tape media in a sequential manner.
In the data processing industry, tape media is known for storing large quantities of data in parallel tracks which extend longitudinally of the tape. The most modern longitudinal tape formats for storing data on tape media, such as magnetic tape cartridges, are described as "serpentine", and have higher track densities by having sets of tracks in both the "out" and "in" directions. For example, the IBM 3570 and IBM 3590 tape drives have 16 "out" tracks, 16 "in" tracks, and 4 "out" and "in" tracks. Access to data is accomplished by indexing the tape heads laterally of the tape media, a process that is very rapid as compared to searching the length of the tape media.
A serpentine longitudinal tape drive records data on a wrap (a track in a single direction) or a set of wraps in one direction along a length of the serpentine longitudinal tape media. Then, the tape drive shifts its recording heads laterally of the tape media a small distance and reverses the tape direction to record another wrap or set of wraps in the opposite direction along the length of the tape media. The tape drive continues these operations back and forth along the serpentine longitudinal tape media until all of blocks of data are written.
Since the physical locations of the data blocks longitudinally along the tape media are staggered back and forth along the tape and not in any sequential order, a straightforward sequential retrieval order of access to the data blocks is most likely not the optimal order of retrieval. Indeed, for data recorded on tape media in a serpentine pattern, the retrieval of blocks of data spaced relatively far apart on the tape will likely result in grossly sub-optimal performance, if the blocks of data are retrieved in sequential order with respect to the order they were written.
Various techniques and systems exist for recording data on and retrieving data from a tape media in a serpentine pattern.
The speed of initial access to the data stored in a serpentine pattern is relatively slow, however.
In tape drives for data stored in a serpentine pattern on a tape media, the tape head typically comprises both servo and data heads which are parallel and spaced apart a certain amount. A servo system in the tape drive employs the servo heads to follow servo tracks on the tape media, while the data heads read and/or write data on the data tracks. Also, when the serpentine data tape media is in a cartridge, a period of time is required for loading the tape cartridge into the drive. Once the cartridge is loaded, an additional period of time is required to thread or move the tape media so that it is in proximity to the tape head. Then, a further period of time is required for the tape drive servo arrangement to initialize the positioning of the tape media with respect to the tape head. This comprises moving the tape longitudinally while the servo arrangement adjusts the lateral positioning of the tape head so that the servo head is properly aligned with the desired servo track and the data head(s) is aligned with the corresponding data wrap (or group of wraps).
In a single reel tape cartridge the device block map is typically at the beginning of the tape. Thus, after the tape leader has been loaded by threading, and the servo head has been properly aligned, the tape data head reads the device block map and a processor processes the device block map in view of the incoming data retrieval request to optimize the retrieval order.
In a reel-to-reel tape cartridge (or cassette), the device block map is typically at the midpoint of the length of tape and the cartridge is stored with an equal amount of tape on each reel. Thus, after the tape or tape head has been moved, typically in a direction normal to the surface of the tape, into proximity to each other, and the servo head has been properly aligned, the tape data head reads the device block map and a processor processes the device block map in view of the incoming data retrieval request to optimize the retrieval order.
If the cartridge is stored in an automated tape library, such as an IBM 3575 Tape Library Dataserver, a still further amount of time is required to retrieve the cartridge from its storage slot and move the cartridge and deliver it to the tape drive.